1. Field of the Invention
This invention relates to a thermal duplicating apparatus and in particular to such a thermal duplicating apparatus which produces a duplicate image on a heat sensitive recording medium with the use of a light ray such as infrared light.
2. Description of the Prior Art
As a thermal duplicating apparatus employing infrared light, an apparatus for producing stencils for use in mimeography is well-known. One example of such a stencil producing apparatus is shown in FIG. 1. As shown, the stencil producing apparatus includes a rotatably supported glass cylinder 1 and a pressure rubber roller 2 disposed in parallel with the cylinder 1. Inside of the cylinder 1 is disposed an infrared light lamp 3 extending in the axial direction of the cylinder 1. A mirror 4 is also disposed inside of the cylinder 1 such that the light emitting from the lamp 3 is collected in the vicinity of the nip line between the cylinder 1 and the roller 2. There is provided a guide member 5 for guiding the advancement of the laminate 6, which is a combination of a stencil and an original, after irradiation of infrared light.
In operation, the pressure roller 2 is driven to rotate in the direction indicated by the arrow 7 to rotate the cylinder 1 in the direction indicated by the arrow 8, and the lamp 3 is lit. Under the condition, the laminate 6 of a stencil and an original is inserted from left into the nip line between the cylinder 1 and the roller 2. As shown in FIG. 2a, the stencil includes a sheet 10 of paper to which is adhered a film 12 by an adhesive agent 11 containing a coloring agent with a separating agent 13 applied to the surface of the film 12. The stencil is brought into intimate contact with an original 14 such that the surface of the stencil having thereon a layer of separating agent 13 faces that side of the original 14 having thereon an original image 15. And then such a laminate is fed into the duplicating apparatus such that infrared light is irradiated to the laminate from its side where the sheet 10 of paper is exposed.
When irradiated by infrared light along the contact line between the cylinder 1 and the roller 2, there appears a difference in the amount of heat absorption between the image area 15 and background area of the original 14 in accordance with a difference in contrast therebetween. As a result, as shown in FIG. 2b, the separating agent 13 and the film 12 melt at those locations opposite to the image area 15 where the amount of heat absorption is large thereby holes 17 are formed and those portions of the coloring agent located in the periphery of the holes 17 become colored. In this manner, holes 17 are formed corresponding to the original image 15 in the surface of the stencil which is transported in the direction indicated by the arrow 9 as guided by the guide member 5. Thus there is obtained a stencil ready to be used in mimeography when the original 14 is peeled off.
However, in the prior art apparatus of FIG. 1, the glass cylinder 1 is relatively thick, ranging from 1.5 to 2 mm, so that the thermal efficiency of the infrared light lamp 3 tends to be lower. Moreover, since the roller 2 is provided as pressed thereagainst, the cylinder 1 could be broken.